There are numerous advantages to using single mode fibres in optical communication systems, but as the global demand for bandwidth increases new ways to encode data are sought. Thus amplitude, phase, wavelength and polarisation are currently employed for data encoding and, despite the advantages of single mode fibre, potentially optical spatial mode division multiplexing using multimode fibre provides a potential further method of increasing the data capacity in future telecommunication systems.
In a single mode fibre (SMF) communication system only light of the fundamental linear polarised (LP) mode, that is the LP01 mode is able to propagate, broadly speaking because the fibre has a small core diameter. Multimode fibre (MMF) has a larger core diameter and multiple light beams of different cross-sectional spatial profiles or modes can propagate simultaneously and the different modes may be employed for different communications channels to provide mode division multiplexing. More particularly, multimode fibre enables optical signals to propagate at higher order LP modes and, especially in combination with wavelength division multiplexing (WDM) and optionally the use of dual-polarisation modulation, this can significantly increase the capacity of the optical telecommunications.
The use of static (non-reconfigurable) optical mode division multiplexing has previously been described in R. Ryf, et al., “Mode-Division Multiplexing Over 96 km of Few-Mode Fiber Using Coherent 6×6 MIMO Processing” J. Light. Technol. 30, 521-531 (2012). However this paper describes a lab-based proof of principle for mode division multiplexing and is not concerned with the solution of practical “in the field” problems. The University of Cambridge (where the inventors are located) has described techniques for mode division multiplexing using a computerised hologram, for example in “Holographic mode generation for mode division multiplexing”, Joel Carpenter and Tim Wilkinson, Poster Session 1 (JW2A), Optical Fiber Communication Conference, Los Angeles, Mar. 4-8, 2012, ISBN: 978-1-55752-938-1. However this is technique relatively complex and bulky and improvements are desirable. General background prior art relating to ferroelectric liquid crystal devices can be found in S. T. Warr and R. J. Mears, “Polarisation insensitive operation of ferroelectric liquid crystal devices”, Electron. Lett. 31, 714-716 (1995); and S. T. Warr and R. J. Mears, “Polarisation insensitive diffractive FLC systems”, Ferroelectrics 181, 53-59 (1996).